you help with my black hollyhock? The leaves keep getting rust on
their undersides. I've changed the potting mix twice and sprayed
them, but have had no luck controlling it.
are hundreds of different fungi causing diseases on plants that
are collectively known as rusts. Each rust generally attacks only
a narrow range of plants. For example, fuchsia rust doesn't affect
peach trees and vice versa.
Some rusts are relatively
easy to control, others very difficult, especially if the host plant
(such as your hollyhock) is susceptible to the disease. Changing
the potting mix won't help, as spores causing the disease just drift
in from all around the neighbourhood.
Apart from getting rid
of the plant altogether there are several steps you can take to
try to minimise the problem, though you'll probably always have
it to some extent. In autumn, once all the leaves and stems have
started to die back, prune them off close to the ground and destroy
them - either put them out in the rubbish or burn them. Rake up
all dead leaves from around the plant. In spring, fertilise and
mulch to encourage strong new growth, as weak shoots are more likely
to become diseased. Try to make sure there's plenty of air movement
around the plant. Prune nearby plants back if necessary.
Before the plant becomes
diseased, spray with a protectant fungicide such as Bravo or Fungus
& Mildew Spray. Once symptoms show, try a spray such as Fungus Fighter
or Greenguard. You'll have to spray regularly, according to the
label recommendations, for long-term control. Don't just use one
product all the time - alternate between two or three of the above
for best overall effect. And ideally pick off any affected leaves
as soon as they appear to prevent even more spores being released
to cause further infection.
Gardener, Issue 154, 2004, Page 26
Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.